Para-xylene is an important raw material in the chemical and fiber industries. For example, terephthalic acid derived from para-xylene is used to produce polyester fabrics. Para-xylene is usually separated from a mixture of para-xylene and at least one other C8 aromatic hydrocarbon by either crystallization, adsorptive separation, or a combination of these two techniques.
U.S. Pat. No. 3,392,113 discloses a cyclic process for the separation of a feed mixture of fluid compounds by contacting the feed with a solid sorbent, such as molecular sieves, selective for at least one compound of said feed mixture, and thereafter passing a fluid desorbent into contact with the sorbent to displace the resulting selectively sorbed compound, said desorbent ordinarily containing trace quantities of aromatic and/or oxygenate impurities which undesirably alter the kinetics, or rates of sorption and desorption of the aforesaid process, over a number of sorption-desorption cycles, the method of stabilizing the kinetics by contacting the desorbent with a separate bed of solid sorbent, prior to utilizing the desorbent in the desorption step, to remove said impurities.
U.S. Pat. No. 5,012,038 recognizes the common use of para-diethylbenzene (p-DEB) as a desorbent for the separation of para-xylene from C8 aromatic hydrocarbon mixtures. It is also known that use of p-DEB as the desorbent limits the C9 aromatics in the feed mixture to less than about 0.1 wt %. This requirement is usually met by first distilling the feed in a so-called xylene splitter column. Otherwise, the C9 aromatic hydrocarbons would gradually build up in the desorbent as it is recycled in the process because C9 aromatics are difficult to separate from p-DEB by simple fractionation and the desorbent must be recycled for economic reasons.
U.S. Pat. No. 5,012,038 and other patents such as U.S. Pat. No. 4,886,930; U.S. Pat. No. 5,057,643; U.S. Pat. No. 5,171,922; U.S. Pat. No. 5,177,295; and U.S. Pat. No. 5,495,061 disclose the use of desorbents having higher boiling points than p-DEB to separate para-xylene from a feed mixture having a C9 aromatic hydrocarbon content greater than 0.1 wt %. The C9 aromatics are then separated from the higher boiling desorbent by fractionation. However, despite the benefits provided by the higher boiling adsorbents, p-DEB continues to be a frequently used desorbent for the adsorptive separation of para-xylene.